This invention relates to a converter for converting an analog input voltage into a digital value by a dual slope system, and more particularly to an A-D converter which is adapted so that an output digital value can be changed, as required, with respect to the same analog input voltage.
An A-D converter of the dual slope system is disclosed, for example, in U.S. Pat. Nos. 3,316,547 issued Apr. 25, 1967 and 4,110,746 issued Aug. 29, 1978. At present, an output voltage of a thermocouple is displayed in digital form using such an A-D converter. The thermocouple usually holds a reference junction temperature at 0.degree. C., and when the temperature of an object to be measured is 0.degree. C., the output voltage of the thermocouple becomes 0 V to display 0.degree. C. In the case of displaying the measured temperature in terms of absolute temperature, with the reference junction temperature held at 0.degree. C., it is necessary to add a bias voltage to the thermocouple output so that 273.15 K is displayed at 0.degree. C. To perform this, it is the practice in the prior art to divide a constant voltage by a resistance-type potential divider circuit and select one of the voltage-divided outputs as the bias voltage to the thermocouple output. The resistance-type potential divider circuit has the defect that it requires expensive resistors of high-precision resistance values and unsusceptible to a temperature change.